The present invention relates to a compression autoignition gasoline engine selectively using spark ignition combustion and compression auto (or self) ignition combustion in accordance with engine operating conditions.
In the compression autoignition combustion, combustion starts at a plurality of points in a combustion chamber, so that the combustion speed is high, and the stability in combustion is high even in a lean state as compared to the ordinary spark ignition combustion. Therefore, the compression autoignition combustion makes it possible to improve the fuel consumption, and significantly reduce the amount of NOx in exhaust gases by lowering the combustion temperature with lean air fuel ratio. The amount of NOx can be further reduced with an air fuel mixture of a uniform air fuel ratio prepared by preliminarily mixing fuel and air.
An engine system performing spark ignition combustion in high speed, high load regions and performing compression autoignition combustion in low speed, low and medium load regions is able to achieve high output at the time of high speed and high load, and simultaneous to achieve improvement of the fuel consumption and reduction of NOx at the time of low speed and low or medium load.
However, in an engine system in which the base compression ratio is increased to obtain the temperature and pressure of air fuel mixture adequate for the compression autoignition combustion, the intake air quantity is decreased or the ignition timing is retarded to prevent knocking in the spark ignition combustion, so that the specific output becomes lower.
A Japanese Published Patent Application Kokai No. 10(1998)-252512 shows an engine system performing compression autoignition combustion by providing a valve shutoff period during which intake and exhaust valves are both closed, for intake heating with internal EGR.
To change the opening and closing timings or the amounts of lift of intake and exhaust valves, a mechanical valve actuating mechanism disclosed in a Japanese Published Patent Application Kokai No. 9(1997)-203307 includes a camshaft having a plurality of cams of different profiles, and a hydraulic actuating piston to change the states of rocker arm""s engaging levers confronting the cams, respectively. This valve actuating mechanism requires several combustion cycles from an operation of an oil pressure control valve to an end of changeover to another rocker arm.
To initiate the compression autoignition combustion by heating the intake air with internal EGR (exhaust gas recirculation), it is necessary to securely initiate the compression autoignition combustion by increasing the amount of internal EGR in a combustion cycle next to the last spark ignition combustion cycle. If, in the next combustion cycle, the compression autoignition combustion is incomplete, the in-cylinder temperature and pressure become lower, and the difficulty to initiate the. compression autoignition combustion further grows.
If, in consideration of the slower response of the mechanical valve actuating mechanism, the amount of the internal EGR is increased before a combustion changeover, the possibility of knocking would be increased in the spark ignition combustion.
On the other hand, a delay in increase of the internal EGR quantity would render the compression autoignition combustion unstable, resulting in a decrease in the quantity of heat supplied by the internal EGR which would magnify the difficulty to regain the stable compression autoignition combustion.
In the case of combustion changeover from compression autoignition combustion to spark ignition combustion, a large amount of internal EGR would increase the in-cylinder temperature and thereby increase the possibility of knocking in the spark ignition combustion. A preliminary reduction of the internal EGR quantity to prevent knocking in the spark ignition combustion would render the compression autoignition combustion unstable before the combustion changeover.
Under some engine operating conditions, an intake pressure is varied in a combustion changeover between the spark ignition combustion and compression autoignition combustion. In this case, a change in the pressure also involves a response delay, and the adjustment of the intake pressure takes several cycles. Therefore, a low in-cylinder pressure renders unstable the compression autoignition combustion in changeover from the spark ignition combustion to the compression autoignition combustion, and a high in-cylinder pressure increases the possibility of knocking in the spark ignition combustion in changeover from the compression autoignition combustion to the spark ignition combustion.
Furthermore, the control of combustion changeover to avoid the combustion instability and knocking is influenced by various engine operating conditions at the time of a combustion changeover.
It is therefore an object of the present invention to provide a compression autoignition gasoline engine, and/or its combustion changeover control process, to achieve smooth and stable combustion changeover sufficiently free from instability and knocking between spark ignition combustion and compression autoignition combustion especially in a light load engine operating region.
It is another object of the present invention to provide system and/or process to achieve a stable combustion changeover between spark ignition combustion and compression autoignition combustion while restraining the occurrence of knocking even when a mechanical valve actuating mechanism is employed to vary valve timings.
According to the present invention, a compression autoignition gasoline engine comprises:
an actuating system comprising an in-cylinder fuel injection system to inject fuel directly into a combustion chamber, and an ignition system operative to change over combustion in the combustion chamber between spark ignition combustion and compression autoignition combustion in accordance with an engine operating condition; and
a controlling system connected with the in-cylinder fuel injection system and the ignition system, to perform transient combustion in transition between the spark ignition combustion and the compression autoignition combustion. The transient combustion may be performed by temporarily changing a fuel injection timing. The transient combustion may be performed by injecting fuel directly into a combustion chamber during an intake valve closing period during which an intake valve is closed.
In illustrated embodiments of the present invention, the transient combustion is one of stratified charge combustion and combustion performed by injecting fuel during a valve shutoff period during which intake and exhaust valves are both closed.
The controlling system comprises a control unit or controller which may include, as a main component, a CPU forming a computer. The controlling system may further comprise one or more input devices such as sensors for collecting input information on one or more engine operating conditions.
A combustion changeover process according to the present invention, for a compression autoignition gasoline engine having an in-cylinder fuel injector, comprises: requesting a combustion changeover from first combustion which is one of spark ignition combustion and compression autoignition combustion to second combustion which is the other of the spark ignition combustion and the compression autoignition combustion, in accordance with an engine operating condition; and performing the transient combustion when the combustion changeover is requested.